The invention relates to a process for the production of petroleum coke suitable for use in the manufacture of carbon anodes employed in Hall-Heroult cells for aluminum manufacture.
Petroleum coke is the residue resulting from the thermal decomposition or pyrolysis of high boiling hydrocarbons at temperatures in the range of about 350.degree. C. to 500.degree. C. High boiling virgin or cracked petroleum residues are typical charging stocks for the production of anode grade coke, the process often being carried out as an integral part of the overall petroleum refinery operation. Industrial petroleum coke is manufactured by methods well known in the art, the major source being the delayed coking process. Other conventional coking methods known in the art include fluid coking and moving bed coking.
Aluminum is conventionally produced in Hall-Heroult cells by the electrolysis of alumina in molten cryolite. The cell is typically a shallow vessel, with a carbon floor forming the cathode, the side walls comprising a rammed coal-pitch or coke-pitch mixture, and the anode consisting of a carbonaceous block suspended in the molten cryolite bath at an anode-cathode separation distance of a few centimeters. The anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon. The cathode is conventionally formed from a pre-baked blend of pitch and calcined anthracite or coke, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom of the cathode.
One of the major requirements of calcined petroleum coke used in the production of carbon anodes is low metallic impurities. The availability of good quality feedstocks for the production of anode grade coke is rapidly diminishing, resulting in increased usage of lower grade crude oils. Increases in the metallic impurities content of the cokes produced from such crude oils can thus be expected because the impurities concentrate in the coke during coking operations.
High levels of metallic impurities, particularly vanadium and nickel, in cokes adversely affect the performance of anodes formed from the cokes. Both the vanadium and nickel in the coke catalyze oxidation of the anode surface exposed to the atmosphere during high temperature cell operation, resulting in air-burning associated with corrosive effects on aluminum cell exhaust duct work. Although some aluminum producers attempt to protect the exposed anode surface by coating with aluminum or burying with alumina after positioning of the anode in the cell, some air-burning still occurs.
The oxidation of petroleum coke by reaction with air at high temperature may be measured in the laboratory by a procedure known in the art as air reactivity.